1. Field of the Invention
This invention relates to the field of optical polymers, and more particularly to highly-halogenated polymers which are useful for fabricating optical waveguides, including optical fibers and integrated optical or xe2x80x9cplanarxe2x80x9d optical devices.
2. Technical Background
Optical waveguides normally consist of a core surrounded by a cladding having a refractive index which is slightly lower than the refractive index of the waveguide core. Glass has been a preferred material for fabrication of optical waveguides, especially optical fibers used to transmit light signals over long distances, because of the excellent mechanical properties of glass and because of the low optical loss of glass materials. However, the use of polymeric materials in the fabrication of optical waveguides has been extensively studied because polymers are lighter in weight than glass, more resistant to impact than glass, do not require surface grinding, and can be more easily mass-produced than glass waveguides.
The polymers that have been frequently used for optical waveguide cores and claddings include homopolymers and copolymers of methylmethacrylate, styrene and esters of methacrylic acid with aliphatic alcohols sometimes containing fluorine, and copolymers of vinylidine fluoride with other monomers containing fluorine. Polymers which have been frequently used for waveguide cladding include homopolymers and copolymers of methacrylates of alcohols containing fluorine, and copolymers of vinylidene fluoride with other monomers containing fluorine.
A problem with most organic polymers is that higher harmonics of carbon-hydrogen bond stretching absorb light in a near-infrared region from about 1000 nm to about 1600 nm. This region encompasses the wavelengths of light signals which are commonly used in optical communication. Accordingly, an undesirable loss of optical signal is associated with the use of most polymer materials for optical waveguides. In order to lower absorption losses of organic polymers in this region, deuterium or fluorine have been used to replace hydrogen atoms bonded to carbon in polymers. However, the incorporation of deuterium is not cost effective and only serves to shift the wavelength at which light is absorbed, but does not totally eliminate absorption of light signals used in optical communication. The use of fluorine also has several disadvantages. High loading of fluorine can lower the refractive index to a level that renders the material incompatible with standard optical fibers, which have refractive indices of about 1.44 at 1550 nm. Another disadvantage is that highly fluorinated polymers do not adhere to other materials. This makes it difficult to join a waveguide cladding to a waveguide core or a substrate for a planar optical device, or to attach input and output fibers to the planar optical device. Also, substitution of fluorine for hydrogen in many polymers results in a polymer which is unacceptably soft, e.g., a polymer having an undesirably low glass transition temperature, melting point temperature, and/or modulus. A further disadvantage with many of the known fluorinated polymers is that they can have a tendency to become cloudy due to crystallization of fluorine-containing components, e.g., vinylidene fluoride and tetrafluoroethylene monomeric units.
Therefore, there is a need for optical polymers which exhibit very low optical loss in the range of wavelengths commonly used for optical communication (e.g., 1000 nm to 2000 nm) and which have a higher glass transition temperature, melting point temperature, and modulus than known highly-halogenated optical polymers.
The invention pertains to the provision of a highly-halogenated optical polymer which exhibits very low optical loss in the range of wavelength commonly used for optical communication (e.g., about 1000 nm to about 2000 nm), and which in accordance with certain preferred embodiments exhibits improved mechanical properties, such as a higher melting point temperature, and a higher modulus than known highly-halogenated polymers used in optical communications. The polymers of this invention are amorphous film forming polymers that have a relatively low variance of refractive index with temperature. This invention also pertains to the use of such polymers as waveguide core and/or waveguide cladding materials in optical fibers and/or planar optical devices.
The optical polymer of this invention are highly-halogenated polyesters, including highly-halogenated polycarbonates. Specific embodiments include polyesters of a highly-halogenated dicarboxylic acid or a derivative of a highly-halogenated dicarboxylic acid and a polyol, and polycarbonates derived from a carbonic acid derivative and a highly-halogenated polyol.